1. Field of the Invention
The present invention relates to an ATM (Asynchronous Transfer Mode) communication system for communicating and switching user information and, more particularly, to an ATM communication system having a cell processing function of effectively performing various processing operations of a cell communicated through an ATM network.
2. Description of the Related Art
In recent years, techniques of ATM (Asynchronous Transfer Mode) communication systems for performing a switching communication of information in an ATM have been developed. The ATM communication system has a variable band feature, and can systematically handle all the communication media having various traffic and service characteristics. In addition, the ATM communication system has been receiving a lot of attention since it can realize high-grade communication services and can customize the communication services in an intelligent network.
In an ATM communication, information is input/output to/from a terminal via a virtual channel (VC), and switching information between nodes or links are performed in units of virtual passes (VPs) each bundling some VCs. Standard specifications of the VCs and VPs are predetermined by the CCITT (International Telegraph and Telephone Consultative Committee), and when an ATM communication system is realized, information communications must be controlled to meet these specifications. For example, a cell flow is observed so that a flow of cells carrying information falls within a bandwidth determined as a VP, and various processing control operations for cells such as control for discarding an overflow cell, must be performed. The standard format of a cell is defined to some extent by the CCITT. However, how to reliably seize asynchronously transmitted cells and how to perform this processing control pose problems.
In particular, problems of how to realize a processing function for a cell flow as hardware, and how to efficiently reduce the hardware scale to a practicable level are left unsolved.
In this manner, when an ATM communication system is realized, various problems associated with hardware remain unsolved, and a strong demand has arisen for solutions to these problems. In particular, serious problems remain unsolved about how to realize, as hardware, cell processing functions, e.g., a function of supplying a cell sent through a transmission line such as an optical fiber to an ATM switch, and a function of sending a cell switched by the ATM switch onto the transmission line.
In case of discussing how to realize the cell processing functions as hardware, the followings are important.
As well known, the ATM communication system is realized by the following architecture.
A node system for switching a VC (Virtual Channel) set between terminal stations, and a link system for switching a VP (Virtual Path) bandling a lot of VCs are defined.
A transmission network is constructed by connecting the link systems by means of the transmission line. The topology of the transmission network may be, for example, a star topology or a ring topology. The topology of the transmission network is determined by a trade-off between an traffic on the transmission network and a cost required for constructing a transmission network.
The node system is connected to the transmission network for multiplexing the VC from the terminal station to the VP set on the transmission network or for separating the VCs within the VP and transiting those to another VP.
Generally, the VP used in the link system is semi-fixedly set in accordance with instruction from an operation center. In contrast, the VC used in the node system is dynamically set-up and released by call setup/release request from the terminal station.
In the node system, there are a node system for a public network being an arrangement for carriers and a node system for a private network used when a subscriber constructs another communication network, using a public network.
As described above, the ATM network is constructed by a link system, a node system for a public network, and a node system for a private network. It is required for realizing the ATM network with a low cost that an architecture for cell processing function is constructed so as to be able to flexibly accommodate an arrangement of different polishing/shaping functions for each of the above node systems, reliability request, etc.